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Coffered ceiling + basic truss question
2

Coffered ceiling + basic truss question

Coffered ceiling + basic truss question

(OP)
I have a house project with a sloped coffered ceilling.  I dont work for a truss manuf.  I just specify the roof structure to use trusses.  Now, I think the only way to make the sloped coffered celing to work is if we support the trusses at the rafter tails.  Do you do this often?
     /\
    /  \
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  /____\
 /         \  
/|        |\
 |        |

I also see another reply by CJSchwartz (ty for being very active on this forum).  He said that he usually put the girder truss on hip roof about 7' from the exterior wall (set back).  My boss always tell me to go 2:1 ratio (girder to wall:girder to ridge).  Shall I start using this 7' set back with maybe some adjusment so that the girder wont land on a window?  This 7' rule of thumb, is that so you will have enough depth on the girder?  Please advise.  Usually I have to layout the trusses before we even know who they will buy the trusses from so I have nobody to talk to.  I just want to lay the trusses out the way truss manuf. like to do.  Thanks!

http://www.swijetty.com
Sea Water Intake and Jetty Construction

RE: Coffered ceiling + basic truss question

If the sloped part of the coffer starts at the wall, it can be done but you probably will need a bigger heel height, or your top chord will need to be a 2x10 or 2x12 to take the concentrated local bending.  Probably won't work on 30-40 foot spans either.


We use 5' or 7' setbacks to keep the End Jack trusses small and simple (2x4 top and bottom chord). One rule of thumb, the shallower the pitch, the further back you'll need to be.  When the length of the building isn't a nice whole number dimension, we'll space our trusses down from the ridge and makeup the oddball dimension in the girder setback (between 5 and 7 feet) because it's easy to make the end jacks say 5'-3" long. That being said, if the building designer specs a dimension for setback to miss a window header or something, the truss designer should honor that.

For reference, most of the trusses we build are 35psf LL for our area.

RE: Coffered ceiling + basic truss question

I have designed tail bearing trusses if the heel height is not tall enough.  If it is not, I will try to use a 2x10.  The heel height will be determined by the seat cut (bird's mouth) on the 2x10.  

I agree with ctcray.  Also, different manufacturers design different hip end types.  So, depending on the style they use, they may have a different set back rule.  The types of hip ends are typically regional, so it is best to find out what is mostly used in your area.  

RE: Coffered ceiling + basic truss question

(OP)
I always draw the trusses line 2' apart.  Ctcay, you said that you start at the ridge.  What if the ridge is let say 11 ft long, That means if you put the one girder at the end of the ridge, what happen to the other end of the ridge?

http://www.swijetty.com
Sea Water Intake and Jetty Construction

RE: Coffered ceiling + basic truss question

There would be a 1' space between trusses with an 11 foot long ridge.  We also try to place a truss right at the intersection of the hip ridges (even if it forces an odd spacing).  You would still maintain 2' o.c. spacing from the peak down.  Sorry, I'm not trying to answer for ctcray, but, that is what we would do.    

RE: Coffered ceiling + basic truss question

(OP)
CJSchwartz, thanks for the response.  I am really trying to grasp all this.  So it is very interesting to hear that you space the ridge at 12" o.c. for 11 ft long ridge.  I always thought you try to keep it to 24" o.c. to safe cost.  So do you try to space it to 12", 16", 19.2", 24" apart for the ridge so a truss always land at the intersection of the ridge and hip?  So if you have an odd size ridge, there will be a truss in the middle of the ridge that will be spaced oddly?

Now, when I measure the set back, is that from the center of the exterior wall or from the edge of exterior wall?  we only draw lines and we dont show the actual thickness of the 2x.  So the line of truss at the intersection of girder and hip, do you put it exactly at the intersection or put it .75" from the intersection toward the ridge (assuming the truss is 1.5" wide).  

http://www.swijetty.com
Sea Water Intake and Jetty Construction

RE: Coffered ceiling + basic truss question

we'd have a 1' spacing in the main portion of the roof, also. If you have to have an odd spacing somewhere, that keeps the hips on each end identical.  Setback is from outside of exterior wall to face of girder.

Unless the exact location of a truss matters to you, I wouldn't try to be that specific in showing your trusses.  I generally indicate with a double-ended arrow the span direction of the trusses and indicate spacing at 24"oc.  Only if I want a precise location of a girder or a certain minimum number of plys do I draw and dimension a line for the actual truss.  That gives the truss designer (and framer) some latitude in his layout.

RE: Coffered ceiling + basic truss question

(OP)
Ok, I think I understand almost 100% of what you two are saying except this 1' spacing at the main.  Let say you have 21 ft of ridge, wouldnt it be cheaper to space the trusses 2 ft apart from both apex and then you will have 2 trusses in the middle of the ridge that is only 1' apart?

http://www.swijetty.com
Sea Water Intake and Jetty Construction

RE: Coffered ceiling + basic truss question

The cost would be the same.  It doesn't matter where you have the odd spacing (as long as it is between the two hip ridge intersections).  You will still have the same number of trusses.  Here's a picture.  Hope it helps.

\ | | | | || /
 \|_|_|_|_||/
 /| | | | ||\  
/ | | | | || \
    
  2'space 1' space
  

RE: Coffered ceiling + basic truss question

(OP)
OHHHH...  Hahah..  I thought you meant to put the the trusses 1' apart the whole way!  Thanks!

http://www.swijetty.com
Sea Water Intake and Jetty Construction

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